[PiP2] Increase PipTransitionState tests coverage

Adding more unit test cases in PipTransitionStateTest
to improve its unit test coverage as per go/pip-unit-test-coverage

Bug: 394938489
Flag: com.android.wm.shell.enable_pip2
Test: atest WMShellUnitTests:PipTransitionStateTest
Change-Id: I9625b34efeaf519c61e7f2bf3b9e843bed315355